CI: Add test for action.yaml to prevent breakage in main

[joestringer]

PR #133 (commit 218de8b) broke the way that action.yaml runs VMs due to the new pull --cache feature. To discover that this was broken, we had to merge the PR, release a new version of LVH, and update cilium/cilium to invoke the new action.yaml. Furthermore, in order to test the fix, it seems like we also need to merge the PR (not yet knowing if it fixes everything), release a new LVH, and update cilium/cilium to invoke the new action.yaml.

It'd be an easier developer experience with lower likelihood to break things if we could push a branch to this repo with a PR, then run the action.yaml + proposed code in the branch before merging. Note that if we implement this, we should be careful not to expose the ability to write untrusted images to official LVH images if a drive-by contributor submits a PR from a fork.

[joestringer]

For reference, I was able to test a new version of the LVH Go code (NB: not the action.yaml code) by pushing a PR to cilium/cilium with extra steps to manually pull & build a target version of LVH. This did not require merging the PR or releasing a new version of LVH. Perhaps we could make a similar action in this repo based on this idea. See this link:

https://github.com/cilium/cilium/pull/30447/files